1. Field of the Invention
This invention relates generally to the field of polyphase DC motor control circuits, and more particularly to a circuit and method for determining the phase winding combination to energize in a motor such as a disk drive motor prior to startup of the motor.
2. Description of the Related Art
Polyphase dc motors are widely used in fixed disk drive applications in personal computers. Typically, such a drive utilizes a magnetic medium on a flat circular disk to store binary data on the disk. The disk is spun by the polyphase dc motor, commonly called a spindle motor, at a relatively constant speed. One or more magnetic heads supported on an actuator arm are selectively moved radially inwardly or outwardly over the surface of the spinning disk by an actuator voice coil motor to read and write data from and to selective locations on the disk. When power is removed from the spindle motor, for example, on power off of the personal computer or on a power failure, routines are automatically implemented to physically move the heads to a predetermined parked position on or off of a landing zone adjacent the data portion of the disk surface, and the rotation of the disks is stopped. When the drive is off, the position of the actuator arm and heads is thus known.
When power is again restored or the PC is turned on again, the disk drive spindle motor starts. It is desirable that the power consumed and time duration of disk drive startup be minimized. This may be desirable for several reasons, the predominant ones of which are that the user cannot access the drive until the disk is running at constant speed and power consumption should be minimized. However, in order to efficiently start the polyphase dc motor, the correct starting point, i.e. phase combination, for the sequence of winding energizations must be determined. This is typically done by passing a small current sequentially through each of the winding combinations in a predetermined sequence and measuring time it takes for the voltage developed across a common sensing resistor in series with each of the winding combinations to reach a predetermined value. A comparator is then used to determine which winding combination takes the longest time to reach the predetermined voltage value and thus senses the winding combination having the highest reluctance from intersecting the magnetic field generated by the stationary permanent magnet pole pieces of the rotor. Passing drive current through this particular winding combination will provide the most starting torque and therefore will be the optimum combination that should be first energized on motor startup.
Typical disk drive spin motor control systems and circuits are described in U.S. Pat. Nos. 5,294,877, 5,317,243, and 5,866,998, which are hereby incorporated herein by reference in their entirety. Hard disk drive spin motors are typically three phase dc motors which have three sets of stator windings, A, B, and C having sets of series coils connected in a Y configuration. The rotor is typically a four pole device. Electrical current is fed through two legs of the "Y" with the third leg left floating in a series of repetitive combinations to cause the rotor to rotate. A sequencer controls a power stage of transistor switches to connect the windings to the voltage sources in the proper order. The six combinations of winding connections are thus AH,BL; AH,CL; BH,CL; BH,AL; CH,AL; and CH,BL; where A, B, C, indicates the winding leg and the "H" and "L" indicates the high side driver transistor, typically a field effect transistor (FET) switch being turned on and the low side driver FET being turned on.
Voltage developed across an external sensing resistor is typically utilized to determine which winding combination to energize first on motor startup. This sensing resistor is typically in series with the low side driver FET. In order to accurately determine which winding combination for the sequencer to use, this resistor must be highly accurate and dependable. Consequently, the cost of this high precision, high reliability sense resistor is rather high. In addition, the sense resistor is typically separate and distinct from the driver IC chip. A separate comparator circuit is also required for implementing the voltage comparison. This conventional configuration is relatively expensive. Therefore there is a need for a more economical approach to sensing the position of the rotor for startup of polyphase dc motors. In addition, there is a need for a circuit which eliminates the need for a precision resistor and eliminates the need for a comparator circuit without loss of reliability and accuracy.